(1) Field of Invention
The present invention relates to gypsum wallboard and more particularly refers to a method for coating the bond liner surface of the paper cover sheet with a polymeric adhesive composition in a selected pattern, to the product formed thereby, and to gypsum wallboard formed by applying a calcined gypsum slurry to the coated bond liner surface of the paper cover sheet.
(2) Description of the Prior Art
Conventional wallboard manufacture has in the past been based on the theory that the wet and dry bond were one and the same, that the paper-to-core bond was mechanical and crystalline, and that drying calcined the crystalline bond to the point that the bond was essentially destroyed. To prevent this, starch was usually introduced into the core slurry and permitted to migrate to the paper-core interface to protect the gypsum crystals forming the mechanical bond. In contrast to this, it was disclosed in U.S. Ser. No. 799,412 of which this application is a continuation-in-part, that the paper-to-core bond actually proceeds in a two-step fashion, as follows. When the slurry is cast onto the paper cover sheets, and specifically the bond liner thereof, wet or green bonding will occur between the bond liner and the slurry due to hydrogen bonding, unless the paper is treated to prevent such hydrogen bonding. For example, any bond liner sizing tends to reduce such hydrogen bonding, and any film completely coating the liner eliminates the hydrogen bonding altogether. Upon drying the board in conventional high temperature kilns, no matter to what degree wet bonding existed prior to drying, it is all destroyed during drying. Thus, for dry bond to take place, a replacement for the destroyed hydrogen bonds must be introduced. Since the dry bond takes place between the bond liner and the gypsum core, the adhesive must be maintained at the paper-core interface until it cures or sets.
It was generally fund and disclosed in U.S. Ser. Nos. 221,324 and 491,097 that the adhesive must be coated upon the bond liner of the cover sheet in such a manner as to permit and maintain wet bonding of the cover sheet during the casting of the wallboard. This required that the adhesive must be non-film forming until it cures or sets, and must be for that reason applied in a discontinuous manner in order that a substantial area of the bond liner surface remains uncoated. The adhesive must further be one which will not migrate from the paper-core interface during the casting and drying of the wallboard, and yet one which will set or cure during the drying before the wet bond has been completely destroyed. As a result of the interplay of wet bonding and subsequent dry bonding, the wallboard is formed which, when dry, retains the adhesive in a discontinuous or spaced-apart or partially covered pattern at the core-to-paper interface, because of the non-migratory nature of the adhesive.
It was previously found that certain uncooked or raw starches and other adhesives are in fact non-migratory and therefore do not spread to completely cover the surface of the cover sheets, but when applied to selected areas leave substantial areas uncoated and free of the adhesive. Consequently a wallboard paper-to-core bond is formed which is uniformly free of "peelers" and paper "blows".
As disclosed in the prior applications referred to above, it was found that when a slurry of raw starch was coated on a paper cover sheet in a discontinuous pattern and in such a manner that substantial areas of the cover sheet were left uncoated even when dry, that the thus coated cover sheets could be utilized either in the wet or dried stage to form gypsum wallboard by applying an aqueous calcined gypsum slurry which itself did not contain starch to the coated paper cover sheets.
As further disclosed in the copending applications referred to, it was found that polyvinyl alcohol in a water solution can be used in place of starch to make coated paper cover sheets which can subsequently be utilized in making gypsum wallboard. In preparing the coated paper, the polyvinyl alcohol in the form of an aqueous solution is coated on the paper in the same manner as disclosed in the prior applications, that is, by applying the coating to limited areas of the paper cover sheet, while leaving substantial areas uncoated which provide wet bonding when the gypsum slurry is applied to the surface of the coated paper cover sheet.
In preparing adhesive-coated paper having substantial areas free of adhesive it has been found that an excellent product can be produced by providing the starch coatings in the form of discrete and definite "striped" patterns or designs wherein the stripes are substantially parallel to each other. It was found that such patterns could be applied to paper with better control and reproducibility by means of a grooved, resilient applicator roll using a direct roll coater.
As disclosed in copending application U.S. Ser. No. 799,412, that the discontinuous or incomplete coatings can be further improved by incorporating into the aqueous coating composition a small amount of finely ground gypsum either in the dihydrate or in the hemihydrate form, but particularly in the hemihydrate form. The incorporated gypsum material increases the viscosity of the coating composition and permits a better and more highly defined coating to be applied, which provides improved adhesion between the paper cover sheets and the gypsum core.